Mobile communication terminal and control unit

ABSTRACT

When a first base station serving a mobile communication terminal in a standby mode, a first neighbor list is acquired from the first base station and stored in a memory. If the serving base station is changed in a standby mode from the first base station to a second base station, a second neighbor list is acquired from the second base station. When the second base station is serving the mobile communication terminal in a standby mode, the communication quality between the mobile communication terminal and each second peripheral base station listed in the acquired second neighbor list, and between the terminal and each first peripheral base station listed in the stored first neighbor list. Based on the measurement results, a peripheral base station that satisfies a preset condition is selected as a hand-off destination candidate.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2003-030510, filed Feb.7, 2003, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a mobile communication terminalfor use in a cellular mobile communication network system.

[0004] 2. Description of the Related Art

[0005] In a cellular mobile communication network system, a plurality ofbase stations are provided in a service area, and each base stationforms a radio zone called a cell. In each cell, the corresponding basestation is connected by radio to mobile communication terminals. In thissystem, when a mobile communication terminal is turned on,synchronization is established between the terminal and the closest basestation. After the establishment of synchronization, the terminal shiftsto a standby state. In the standby state, an incoming call istransmitted from the base station to the terminal, or negotiation isperformed for establishing a communication link between the terminal andbase station if the terminal performs an operation for an outgoing call.When the communication link is established, the terminal shifts to acommunication state.

[0006] In a cellular mobile communication network system, hand-off isperformed when a mobile communication terminal moves from a cell toanother cell. “Hand-off” means a process for changing, to another, thebase station as a synchronization establishment destination. In thestandby state and communication state, a mobile communication terminalacquires, from the base station with which synchronization isestablished (hereinafter referred to as an “active base station”), aneighbor list message listing the base stations located near the activebase station. The mobile communication terminal searches for theneighboring base stations, using this message, and performs hand-offprocessing, if necessary.

[0007] For example, in a system employing a code division multipleaccess (CDMA) method, in which a mobile communication terminalestablishes synchronization with a base station using slidingcorrelation therebetween, each base station transmits a pilot signaldiffused by a diffusion code. The diffusion code used to diffuse thepilot signal is provided with an initial phase offset value, whichvaries between base stations. Each mobile communication terminalreceives pilot signals transmitted from peripheral base stations, usingthe offset values recited in the neighbor list message, therebymeasuring the reception quality of the pilot signals. The measurement ofreception quality is performed by, for example, detecting thesignal/interference-wave ratio (SIR) or reception-signal level. Based onthe measurement results, the mobile communication terminal selects aperipheral base station of the best reception quality, and uses thisbase station as a destination candidate with which synchronization is tobe established.

[0008] On the other hand, if all the measured reception quality levelsare lower than a reference value, the mobile communication terminaldetermines that there is no appropriate base station for a destinationcandidate. In other words, it is determined that all peripheral basestations are out of range. After that, the terminal searches the basestations over all phases of the diffusion code used to diffuse pilotsignals, and selects the base station from which the best receptionquality can be acquired. Searching over all phases of a diffusion codecorresponds to initial resetting for selecting a base station upon theturn-on of the mobile communication terminal. Accordingly, thisoperation requires a lot of time and power consumption. If it isperformed frequently, the battery life of the terminal is shortened.

[0009] For various reasons, “Out of Range” determination is made. Forexample, the acquired neighbor list may become an inappropriate one dueto a change in the positional relationship between the mobilecommunication terminal and base stations.

[0010]FIG. 4 is a view useful in explaining such a case. In the figure,assume that a mobile communication terminal MS establishessynchronization with a base station BS-A, and the neighbor list acquiredfrom the active base station BS-A lists a peripheral base station BS-B.In this state, even if the terminal MS moves to a second position, thecommunication state does not change. However, if the terminal MS furthermoves to a third position, the communication between the terminal andbase station BS-A is interrupted by an obstacle 1. At this time, theterminal MS hands off from the base station BS-A to the peripheral basestation BS-B listed in the neighbor list.

[0011] After that, if the terminal MS moves to a fourth position,communication with the base station BS-B is interrupted by an obstacle2. The fourth position is close to a base station BS-C. However, if theneighbor list acquired from the base station BS-B does not list the basestation BS-C, and a pilot signal of sufficient reception quality cannotbe acquired from any other peripheral base station listed in theneighbor list, the terminal MS determines that it is out of the range ofthe base stations listed in the neighbor list. In this case, the mobilecommunication terminal MS must perform the above-mentioned searchingover all phases of a diffusion code.

[0012] Since the transmission lines for mobile communication are verycomplex, it is difficult to prepare a neighbor list that enables optimalhand-off processing to be executed in all positions in its service area.Therefore, it is a frequent practice to perform searching over allphases of a diffusion code.

[0013] There are known techniques for reducing the power consumption ofa mobile communication terminal by shortening the time required forbase-station searching for hand-off. Jpn. Pat. Appln. KOKAI PublicationNo. 2002-171555, for example, discloses the following technique (pages 2to 5 and FIG. 1): When a mobile communication terminal selects aperipheral base station as a hand-off destination candidate, it stores,in its database, information indicating the base station, together withpositional information indicating the position of the terminal itself.When the terminal again enters the cell indicated by the positionalinformation, it selects the information, stored in the database, inpreference to a neighbor list acquired from a base station correspondingto the cell. Based on the selected information, the terminal searchesthe peripheral base stations for a hand-off destination candidate. Thissearch technique, however, requires a function for acquiring informationindicating the position of a mobile communication terminal, whichincreases the circuit scale of the terminal and the amount of processingby the CPU of the terminal.

[0014] Further, Jpn. Pat. Appln. KOKAI Publication No. 2001-54156discloses a technique for searching for a pilot signal transmitted froma peripheral base station, acquiring a neighbor list from the basestation detected by searching, and selecting a hand-off destinationcandidate from the acquired neighbor list. In this technique, to acquirea neighbor list from a peripheral base station, a mobile communicationterminal holds synchronization with the active base station, whileperiodically searching peripheral base stations other than the activeone to select an accessible base station. After selecting the accessiblebase station, the terminal must perform processing for acquiring aneighbor list therefrom. This processing significantly reduces thebattery life of the terminal.

BRIEF SUMMARY OF THE INVENTION

[0015] It is an object of the invention to provide a mobilecommunication terminal capable of efficiently selecting a base stationas an appropriate hand-off destination candidate, without acquiringinformation indicating the position of the terminal itself, or neighborlists from peripheral base stations, thereby reducing the powerconsumption of the terminal and increasing its battery life. It isanother object of the invention to provide a base-station selectionmethod employed in this mobile communication terminal.

[0016] To attain the objects, in a mobile communication terminalaccording to an embodiment, a first neighbor list is acquired from thefirst base station serving the mobile communication terminal in astandby mode, and stored in a memory. If the serving base station ischanged in a standby mode from the first base station to a second basestation, a second neighbor list is acquired from the second basestation. When the second base station is serving the mobilecommunication terminal in a standby mode, the communication qualitybetween the mobile communication terminal and each of the secondperipheral base stations listed in the acquired second neighbor list ismeasured. Further, the communication quality between the mobilecommunication terminal and each of the first peripheral base stationslisted in the stored first neighbor list is measured. Based on themeasured communication quality, a peripheral base station that satisfiesa preset condition is selected as a hand-off destination candidate.

[0017] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be leaned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0018] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate an embodiment of theinvention, and together with the general description given above and thedetailed description of the embodiment given below, serve to explain theprinciples of the invention.

[0019]FIG. 1 is a block diagram illustrating the configuration of amobile communication terminal according to a first embodiment of theinvention;

[0020]FIG. 2 is a flowchart illustrating the procedure and contents ofbase-station selecting control performed by the mobile communicationterminal of FIG. 1;

[0021]FIG. 3 is a flowchart illustrating the procedure and contents ofbase-station selecting control performed by a mobile communicationterminal according to a second embodiment of the invention; and

[0022]FIG. 4 is a view useful in explaining problems in prior art.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

[0023] A cellular mobile communication network system according to afirst embodiment of the invention employs a code division multipleaccess (CDMA) scheme. A base station transmits a pilot signal. The pilotsignal is diffused by a predetermined diffusion code. An initial phaseoffset value that varies between base stations is assigned to adiffusion code. When establishing initial synchronization with a basestation upon turn-on or out of synchronization, a mobile communicationterminal searches all peripheral base stations close to the terminal,using a sliding correlator. Further, when establishing synchronizationin preparation for hand-off, the terminal receives pilot signalstransmitted from the active base station with which synchronization isnow established, and from other peripheral base stations, and measuresthe reception quality of the signals.

[0024]FIG. 1 is a circuit block diagram illustrating the configurationof a mobile communication terminal according to the first embodiment.This terminal comprises a radio unit U1, input/output unit U2, controlunit U3 and power unit U4.

[0025] The radio unit U1 has an antenna 1, antenna duplexer 2, receptioncircuit 3, transmission circuit 4 and modulation/demodulation circuit 5.The reception circuit 3 amplifies the low noise component of a radiosignal received by the antenna 1, and down-converts it into anintermediate frequency signal or baseband frequency signal.

[0026] The modulation/demodulation circuit 5 converts a received signalinto a digital signal, and inputs it to a RAKE receiver and searchreceiver. The RAKE receiver has a plurality of finger circuits and asymbol synthesizer. Each finger circuit subjects the input signal todespreading, thereby reproducing a received signal of a desired path.The symbol synthesizer adjusts the phases of signals output from thefinger circuits, and subjects the signal to symbol synthesis. Theresultant signal is decoded.

[0027] The search receiver has a sliding correlator. Using the slidingcorrelator, a diffusion code for a pilot signal transmitted from a basestation is detected by searching, thereby detecting its offset value.The detection result is input to the control unit 10.

[0028] The modulation/demodulation circuit 5 subjects transmission datato orthogonal modulation and diffusion using a diffusion code, thenconverts the resultant signal into an analog signal, and outputs theanalog signal to the transmission circuit 4. The transmission circuit 4up-converts the input analog transmission signal into a radio frequencysignal, and amplifies it. The amplified signal is output to the antenna1 via the antenna duplexer 2, and is transmitted therefrom to a basestation.

[0029] The input/output unit U2 comprises a speech decoder 6, speechcoder 7, speaker 8, microphone 9, display 11 and input device 12. Thedisplay 11 is formed of an LCD, and the input device 12 is formed of adial keypad.

[0030] The speech decoder 6 has a digital/analog converter and driver.The decoder 6 converts, into an analog signal, reception datademodulated by the modulation/demodulation circuit 5, thereby amplifyingit and output it as voice through the speaker 8. The speech coder 7 hasa speech amplifier and analog/digital converter. The coder 7 amplifiesthe speech signal output from the microphone 9, thereby converting itinto transmission data as a digital signal, and supplying it to themodulation/demodulation circuit 5.

[0031] The control unit U3 comprises a central processing unit (CPU) 10and memory 20. The memory 20 is formed of, for example, a flash memoryor RAM. The memory 20 has first and second memory areas 21 and 22 forstoring neighbor lists. Specifically, the first memory area 21 stores aneighbor list NLa acquired from the active base station with whichsynchronization is now established. The second memory area 22 storesneighbor lists NLb and NLc acquired from two base stations with whichsynchronization was established before. The neighbor lists NLb and NLcacquired from the past two base stations are called expanded neighborlists.

[0032] The CPU 10 has a neighbor-list storage control function,base-station selection control function and hand-off control function ascontrol functions for realizing the first embodiment.

[0033] The neighbor-list storage control function acquires a neighborlist NLa from the active base station with which synchronization is nowestablished, each time initial synchronization is established oractive-base-station change occurs as a result of hand-off. Thethus-acquired neighbor list NLa is stored in the first memory area 21.Before storing the newest neighbor list NLa, the neighbor lists NLa andNLb stored in the first and second memory areas 21 and 22 are all storedin the second memory area 22 as expanded neighbor lists NLb and NLc.

[0034] The base-station selection control function reads the neighborlist LNa and expanded neighbor lists NLb and NLc from the first andsecond memory areas 21 and 22, and measures the reception quality of theperipheral base stations listed in the neighbor lists NLa, NLb and NLc.The measurement of reception quality is performed by detecting the SIRsof signals transmitted from the respective peripheral base stations. Themeasured reception quality levels are compared with a preset thresholdvalue. If a certain reception quality level is higher than the thresholdvalue, the peripheral base station corresponding to the receptionquality level is selected and stored as a hand-off destinationcandidate. If there are several reception quality levels higher than thethreshold values, the peripheral base station corresponding to thehighest reception quality level is selected as the hand-off destinationcandidate. In this case, the peripheral base stations corresponding toreception quality levels lower than the highest level and higher thanthe threshold value are stored as substitutes.

[0035] The hand-off control function periodically measures the receptionquality of the active base station with which synchronization is nowestablished, and compares the measured reception quality with a presetmaintenance reference value. If it is determined as a result ofcomparison that the measured reception quality becomes less than themaintenance reference value, the reception quality of a peripheral basestation stored as a hand-off destination candidate is measured andcompared with the maintenance reference value and a preset hand-offreference value. If the former is not less than the hand-off referencevalue or maintenance reference value, the mobile communication terminaland active base station cooperate to perform processing for the terminalto hand off from the active base station to the hand-off destinationcandidate. On the other hand, if the measured reception quality is lessthan the hand-off reference value or maintenance reference value, anunspecified number of base stations existing in all cells are searched,and synchronization is established with a base station of the bestreception quality detected by this searching.

[0036] The power unit U4 comprises a power supply circuit 13 and battery14. From the output of the battery 14, the power supply circuit 13generates a DC voltage needed for operating the units U1, U2 and U3.

[0037] The operation of the mobile communication terminal constructed asabove that is related to hand-off will now be described. FIG. 2 is aflowchart illustrating the control procedure and control contents of theCPU 10.

[0038] The first memory area 21 already stores a neighbor list NLaacquired from the active base station with which synchronization is nowestablished, and the second memory area 22 already stores neighbor listsNLb and NLc acquired from two base stations with which synchronizationwas established before.

[0039] In this state, the CPU 10 measures the reception quality levelsof all to-be-measured base stations at a step 2 a. Firstly, the CPU 10causes the radio unit U1 to receive a pilot signal transmitted from theactive base station with which synchronization is now established, andmeasures the SIR of the received signal. Subsequently, the CPU 10 readsthe neighbor list NLa from the first area 21 of the memory 20, therebycausing the radio unit U1 to receive a pilot signal transmitted fromeach peripheral base station listed in the read neighbor list NLa, andmeasuring the SIR of the signal. Furthermore, the CPU 10 reads theexpanded neighbor lists NLb and NLc from the second area 22 of thememory 20, thereby causing the radio unit U1 to receive a pilot signaltransmitted from each peripheral base station listed in the readneighbor lists NLb and NLc, and measuring the SIR of the signal.

[0040] Thereafter, the CPU 10 selects a peripheral base station as ahand-off destination candidate at a step 2 b. Specifically, the CPUcompares the SIR measured at the step 2 a with a preset threshold value,and selects a peripheral base station of the best reception quality fromthe peripheral base stations having reception quality levels not lessthan the threshold value, thereby storing the selected base station as ahand-off destination candidate.

[0041] When determining whether hand-off processing should be performed,the CPU 10 causes, at a step 2 c, the radio unit U1 to receive a pilotsignal transmitted from the active base station with whichsynchronization is now established, measures the SIR of the receivedsignal, and compares the measured SIR with a preset maintenancereference value. If it is determined as a result of comparison that themeasured SIR is not less than the maintenance reference value, theprocessing is returned to the standby processing. If, on the other hand,the measured SIR becomes less than the maintenance reference value, theCPU 10 measures, at a step 2 d, the SIR of a pilot signal from theperipheral base station stored as the hand-off destination candidate,and compares the measured SIR with the preset hand-off reference value.If the measured SIR is not less than the hand-off reference value, theCPU 10 proceeds to a step 2 j where it performs hand-off processing.Hand-off processing is executed when the CPU issues a hand-off requestto the active base station with which synchronization is nowestablished, and receives an instruction to perform hand-off processing,issued from the active base station in response to the hand-off request.The hand-off request contains identification information indicating aperipheral base station as a hand-off destination candidate.

[0042] If, on the other hand, the SIR of a pilot signal from theperipheral base station as the hand-off destination candidate is lessthan the hand-off reference value, the CPU 10 proceeds to a step 2 ewhere the measured SIR is compared with the maintenance reference value.If the measured SIR is not less than the maintenance reference value,the CPU 10 proceeds to the step 2 j, thereby performing theabove-mentioned hand-off processing.

[0043] If the measured SIR is less than the maintenance reference value,the CPU 10 proceeds to a step 2 f, thereby searching an unspecifiednumber of base stations existing in all cells, and selecting the basestation of the highest SIR. At a step 2 g, the CPU 10 establishessynchronization with the selected base station.

[0044] After establishing synchronization with the new base station, theCPU 10 proceeds to a step 2 h where it acquires a neighbor list NLa fromthe new active base station. At a step 2 i, the CPU 10 updates theneighbor list NLa stored in the first memory area 21, and the expandedneighbor lists NLb and NLc stored in the second memory area 22.

[0045] The acquisition of a neighbor list NLa from a new active basestation (step 2 h), and the update of the neighbor lists NLa, NLb andNLc in the memory 20 (step 2 i) are also performed after the executionof the hand-off processing (step 2 j).

[0046] As described above, in the first embodiment, each time thepresent active base station as a synchronization destination is changedto a new one, a neighbor list NLa is acquired from the present activebase station and stored in the first memory area 21 of the memory 20. Atthe same time, two, at maximum, of the neighbor lists, stored so far inthe first and second memory areas 21 and 22, are then stored in thesecond memory area 22. When a peripheral base station as a hand-offdestination candidate is selected, the neighbor list NLa and expandedneighbor lists NLb and NLc are read from the first and second memoryareas 21 and 22, and the reception quality levels (SIRs) of theperipheral base stations listed in these lists are measured. Based onthe measurement results, the peripheral base stations having SIRs higherthan the threshold value are selected as hand-off destinationcandidates.

[0047] Thus, the selection targets as hand-off destination candidatesare not limited to the peripheral base stations listed in the neighborlist NLa acquired from the present active base station, but are expandedto those listed in the neighbor lists NLb and NLc acquired from the pasttwo active base stations. Accordingly, the probability of selecting anappropriate peripheral base station is increased. This being so, even ina place, such as a city or mountain place, in which the transmissionline environment easily varies, the degree of necessity of searching allcells is lowered, which suppresses the power consumption of mobilecommunication terminals and increases the lives of batteries.

[0048] Further, to expand the selection target range of hand-offdestination candidates, it is not necessary to acquire positioninformation concerning the mobile communication terminal, or to acquirea neighbor list from a peripheral base station other than the activebase station. Therefore, the power used for these purposes can be saved.

Second Embodiment

[0049] In a second embodiment of the invention, when selecting aperipheral base station as a hand-off destination candidate, firstly,the peripheral base stations listed in the neighbor list NLa stored inthe first memory area 21 are subjected to reception quality measurement.If no appropriate hand-off destination candidate is detected from themeasurement results, the reception quality levels of the peripheral basestations listed in the expanded neighbor lists NLb and NLc are measuredto select a hand-off destination candidate.

[0050]FIG. 3 is a flowchart illustrating the procedure and contents ofbase-station selecting control performed by a mobile communicationterminal according to the second embodiment of the invention. Since themobile communication terminal of the second embodiment has the sameconfiguration as that shown in FIG. 1, the configuration of the secondembodiment will be described with reference to FIG. 1.

[0051] The first memory areas 21 already stores a neighbor list NLaacquired from the active base station with which synchronization is nowestablished, and the second memory area 22 already stores neighbor listsNLb and NLc acquired from two base stations with which synchronizationwas established before.

[0052] In this state, the CPU 10 causes the radio unit U1 to receive apilot signal transmitted from the active base station with whichsynchronization is now established, and measures the SIR of the receivedsignal (step 3 a). Subsequently, the CPU 10 reads the neighbor list NLafrom the first area 21 of the memory 20, thereby causing the radio unitU1 to receive a pilot signal transmitted from each peripheral basestation listed in the read neighbor list NLa, and measuring the SIR ofthe signal.

[0053] At a step 3 b, the CPU 10 compares the measured SIR with a presetthreshold value, selects a peripheral base station of the best receptionquality from the peripheral base stations having SIRs higher than thethreshold value, and stores it as a hand-off destination candidate.

[0054] When determining whether hand-off processing should be performed,the CPU 10 causes, at a step 3 c, the radio unit U1 to receive a pilotsignal transmitted from the active base station with whichsynchronization is now established, measures the SIR of the receivedsignal, and compares the measured SIR with a preset maintenancereference value. If it is determined as a result of comparison that themeasured SIR is not less than the maintenance reference value, theprocessing is returned to the standby processing. If, on the other hand,the measured SIR becomes less than the maintenance reference value, theCPU 10 measures, at a step 3 d, the SIR of the peripheral base stationstored as the hand-off destination candidate, and compares the measuredSIR with a preset hand-off reference value. If the measured SIR is notless than the hand-off reference value, the CPU 10 proceeds to a step 3n where it performs hand-off processing.

[0055] If, on the other hand, the SIR of the peripheral base station asthe hand-off destination candidate is less than the hand-off referencevalue, the CPU 10 proceeds to a step 3 e where the measured SIR iscompared with the maintenance reference value. If the measured SIR isnot less than the maintenance reference value, the CPU 10 proceeds tothe step 3 n, thereby performing hand-off processing.

[0056] If the measured SIR is less than the maintenance reference value,the CPU 10 proceeds to a step 3 f, where it reads the expanded neighborlists NLb and NLc from the second memory area 22 of the memory 20,thereby causing the radio unit U1 to receive pilot signals from theperipheral base stations listed in the read neighbor lists, andmeasuring the SIR of each received signal.

[0057] Subsequently, the CPU 10 compares the measured SIR with a presetthreshold value, thereby selecting a peripheral base station of the bestreception quality from the peripheral base stations having receptionquality levels higher than the threshold value, and storing it as ahand-off destination candidate (step 3 g).

[0058] Thereafter, the CPU 10 proceeds to a step 3 h, where it compares,with a maintenance reference value, the measured SIR of the peripheralbase station selected as a hand-off destination candidate from theexpanded neighbor lists NLb and NLc. If it is determined as a result ofcomparison that the measured SIR is not less than the maintenancereference value, the CPU 10 proceeds to the step 3 n, thereby executinghand-off processing.

[0059] On the other hand, if the measured SIR is less than themaintenance reference value, the CPU 10 proceeds to a step 3 i, therebysearching an unspecified number of base stations existing in all cells.From the search results, the CPU 10 selects a base station correspondingto the highest SIR. At the nest step 3 j, the CPU 10 establishessynchronization with the selected base station.

[0060] Upon establishing synchronization with the new base station, theCPU 10 acquires a neighbor list NLa from the new active base station ata step 3 k. At the next step 3 m, the CPU 10 updates the neighbor listNLa stored in the first memory area 21 of the memory 20, and theexpanded neighbor lists NLb and NLc stored in the second memory area 22.

[0061] The acquisition of a neighbor list NLa from a new active basestation (step 3 k), and the update of the neighbor lists NLa, NLb andNLc in the memory 20 (step 3 m) are also performed after the executionof the hand-off processing (step 3 n).

[0062] As described above, in the second embodiment, when selecting aperipheral base station as a hand-off destination candidate, firstly,the reception quality of each peripheral base station listed in theneighbor list NLa stored in the first memory area 21 is measured. If aperipheral base station that satisfies the condition is not found, thereception quality of each peripheral base station listed in the expandedneighbor lists NLb and NLc stored in the second memory area 22 ismeasured to select the hand-off destination candidate.

[0063] As described above, only when an appropriate one as a hand-offdestination candidate is not found in the peripheral base stationslisted in the neighbor list NLa acquired from the active base stationwith which synchronization is now established, the peripheral basestations listed in the expanded neighbor lists NLb and NLc are searched.Therefore, the power required for searching peripheral base stations canbe reduced and hence the battery life can be further lengthened,compared to the case where all peripheral base stations listed in theneighbor list NLa and expanded neighbor lists NLb and NLc aresimultaneously searched. This also leads to the shortening of the timerequired to select a peripheral base station as a hand-off destinationcandidate (i.e., the required hand-off processing time).

Other Embodiments

[0064] In the above-described embodiments, all peripheral base stationslisted in the neighbor list NLa and expanded neighbor lists NLb and NLcare searched unconditionally. However, the peripheral base stationslisted in the neighbor list NLa may overlap those listed in the expandedneighbor lists NLb and NLc. Further, the peripheral base stations listedin the expanded neighbor list NLb may overlap those listed in theexpanded neighbor list NLc. In light of this, when searching theperipheral base stations listed in the expanded neighbor lists NLb andNLc, the overlapping ones are excluded so that only the different onesare searched.

[0065] This prevents the same peripheral base station from beingsearched doubly, thereby enabling an appropriate peripheral base stationas a hand-off destination candidate to be selected by a smaller numberof searching operations and hence in a shorter time (the requiredhand-off processing time can be reduced). In other words, the powerrequired for searching peripheral base stations can be further reduced,and the battery life can be further lengthened.

[0066] Further, priority levels may be set for the peripheral basestations listed in the expanded neighbor lists. In this case, theperipheral base stations are searched in the order of priority. Eachtime one peripheral base station is searched, the SIR of the station maybe compared with a threshold value, and if the SIR is not less than thethreshold value, this peripheral base station may be instantly selectedas a hand-off destination candidate. The priority level can be set basedon, for example, the frequency of searching operations, receptionquality, the number of occasions in which the peripheral base stationhas been selected as a hand-off destination candidate, and the number ofoccasions in which the peripheral base station has been selected as ahand-off destination.

[0067] Furthermore, a plurality of search lists of different prioritylevels may be prepared. In this case, a searching operation isperformed, beginning from the peripheral base stations listed in asearch list of the highest priority level, thereby selecting oneperipheral base station as a hand-off destination candidate.

[0068] This can reduce the number of searching operations performed toselect a peripheral base station as a hand-off destination candidate,thereby further reducing the required hand-off processing time and powerconsumption, and lengthening the battery life.

[0069] In the above-described embodiments, a neighbor list NLa acquiredfrom an active base station is stored as an expanded neighbor list untiltwo more neighbor lists NLa are newly acquired. However, the neighborlist NLa may be stored until a number n (n≧3) of more neighbor lists arenewly acquired. Yet further, the expanded neighbor lists may be storedfor a preset time, and erased after the present time elapses. Thisenables the memory to store only relatively new neighbor lists.

[0070] In addition, in the above embodiments, the SIR of a pilot signalis measured. However, the quality of communication may be measured froma power control signal contained in data transmitted from a base stationvia a traffic channel, or from the received electric field intensity(RSSI).

[0071] The present invention is also applicable to a mobilecommunication terminal that employs a communication scheme, other thanCDMA in which initial synchronization is established using a slidingcorrelator.

[0072] The circuit configuration of the mobile communication terminal ofthe invention, and the procedure and contents of neighbor listacquisition control, base-station selection control and hand-off controlemployed in the invention may be modified in various ways withoutdeparting from the scope of the invention.

[0073] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A mobile communication terminal for use in acellular mobile communication system, comprising: a circuit configuredto acquire a first neighbor list from the first base station serving themobile communication terminal in a standby mode, the first neighbor liststoring data indicating first peripheral base stations existing near thefirst base station; a memory configured to store the acquired firstneighbor list; a circuit configured to acquire, if the serving basestation is changed in a standby mode from the first base station to asecond base station, a second neighbor list from the second basestation, the second neighbor list storing data indicating secondperipheral base stations existing near the second base station; ameasurement circuit configured to measure, when the second base stationis serving the mobile communication terminal in a standby mode,communication quality between the mobile communication terminal and eachof the second peripheral base stations listed in the acquired secondneighbor list, and communication quality between the mobilecommunication terminal and each of the first peripheral base stationslisted in the stored first neighbor list; and a circuit configured toselect, as a hand-off destination candidate, one of the first peripheralbase stations and the second peripheral base stations, which satisfies apreset condition, based on the measured communication quality.
 2. Themobile communication terminal according to claim 1, wherein the memorystores the first neighbor list until a number of occasions in whichselection for selecting the hand-off destination candidate is performedreaches a preset value.
 3. The mobile communication terminal accordingto claim 1, wherein the memory stores the first neighbor list for apreset time.
 4. The mobile communication terminal according to claim 1,wherein the measurement circuit measures reception quality of a pilotsignal transmitted from each of the first and second peripheral basestations.
 5. The mobile communication terminal according to claim 1,wherein the measurement circuit measures the communication qualitybetween the mobile communication terminal and each of the secondperipheral base stations listed in the acquired second neighbor list,the measurement circuit also measuring the communication quality betweenthe mobile communication terminal and those of the first peripheral basestations listed in the stored first neighbor list, which are obtained byexcluding the first peripheral base stations doubly listed as the secondperipheral base stations in the second neighbor list.
 6. A mobilecommunication terminal for use in a cellular mobile communicationsystem, comprising: a circuit configured to acquire a first neighborlist from the first base station serving the mobile communicationterminal in a standby mode, the first neighbor list storing dataindicating first peripheral base stations existing near the first basestation; a memory configured to store the acquired first neighbor list;a circuit configured to acquire, if the serving base station is changedin a standby mode from the first base station to a second base station,a second neighbor list from the second base station, the second neighborlist storing data indicating second peripheral base stations existingnear the second base station; a first measurement circuit configured tomeasure, when the second base station is serving the mobilecommunication terminal in a standby mode, communication quality betweenthe mobile communication terminal and each of the second peripheral basestations listed in the acquired second neighbor list; a first selectioncircuit configured to select, as a hand-off destination candidate, oneof the second peripheral base stations, which satisfies a presetcondition, based on the measured communication quality between themobile communication terminal and each of the second peripheral basestations; a second measurement circuit configured to measure thecommunication quality between the mobile communication terminal and eachof the first peripheral base stations listed in the first neighbor list,if the second peripheral base stations do not satisfy the presetcondition; and a second selection circuit configured to select, as thehand-off destination candidate, one of the first peripheral basestations, which satisfies the preset condition, based on the measuredcommunication quality between the mobile communication terminal and eachof the first peripheral base stations.
 7. The mobile communicationterminal according to claim 6, wherein the memory stores the firstneighbor list until a number of occasions in which selection forselecting the hand-off destination candidate is performed reaches apreset value.
 8. The mobile communication terminal according to claim 6,wherein the memory stores the first neighbor list for a preset time. 9.The mobile communication terminal according to claim 6, wherein thefirst and second measurement circuits measure reception quality of apilot signal transmitted from each of the first and second peripheralbase stations.
 10. The mobile communication terminal according to claim6, wherein the second measurement circuit measures the communicationquality between the mobile communication terminal and those of the firstperipheral base stations listed in the stored first neighbor list, whichare obtained by excluding the first peripheral base stations doublylisted as the second peripheral base stations in the second neighborlist.
 11. The mobile communication terminal according to claim 6,further comprising a search circuit configured to search, if the firstperipheral base stations do not satisfy the preset condition, anunspecified number of peripheral base stations for a peripheral basestation having communication quality which satisfies the presentcondition.
 12. A control unit incorporated in a mobile communicationterminal for use in a cellular mobile communication system, the mobilecommunication terminal also incorporating a radio unit configured totransmit and receive radio signals to and from base stations, the radiounit being connected to the control unit, the control unit comprising: afirst reception control section configured to make the radio unit toreceive a first neighbor list from the first base station serving themobile communication terminal in a standby mode, the first neighbor liststoring data indicating first peripheral base stations existing near thefirst base station; a memory configured to store the received firstneighbor list; a second reception control section configured to make theradio unit to receive, if the serving base station is changed in astandby mode from the first base station to a second base station, asecond neighbor list from the second base station, the second neighborlist storing data indicating second peripheral base stations existingnear the second base station; a measurement control section configuredto measure, when the second base station is serving the mobilecommunication terminal in a standby mode, communication quality betweenthe mobile communication terminal and each of the second peripheral basestations listed in the acquired second neighbor list, and communicationquality between the mobile communication terminal and each of the firstperipheral base stations listed in the stored first neighbor list,measurement of the communication quality being performed based on thesignals received by the radio unit; and a selection section configuredto select, as a hand-off destination candidate, one of the firstperipheral base stations and the second peripheral base stations, whichsatisfies a preset condition, based on the measured communicationquality.
 13. A control unit incorporated in a mobile communicationterminal for use in a cellular mobile communication system, the mobilecommunication terminal also incorporating a radio unit configured totransmit and receive radio signals to and from base stations, the radiounit being connected to the control unit, the control unit comprising: afirst reception control section configured to make the radio unit toreceive a first neighbor list from the first base station serving themobile communication terminal in a standby mode, the first neighbor liststoring data indicating first peripheral base stations existing near thefirst base station; a memory configured to store the received firstneighbor list; a second reception control section configured to make theradio unit to receive, if the serving base station is changed in astandby mode from the first base station to a second base station, asecond neighbor list from the second base station, the second neighborlist storing data indicating second peripheral base stations existingnear the second base station; a first measurement control sectionconfigured to measure, when the second base station is serving themobile communication terminal in a standby mode, communication qualitybetween the mobile communication terminal and each of the secondperipheral base stations listed in the acquired second neighbor list,based on the signal received by the radio unit; a first selectionsection configured to select, as the hand-off destination candidate, oneof the second peripheral base stations which satisfies a presetcondition, based on the communication quality measured by the firstmeasurement control section; a second measurement control sectionconfigured to measure, if the second peripheral base stations do notsatisfy the preset condition, communication quality between the mobilecommunication terminal and each of the first peripheral base stationslisted in the stored first neighbor list, based on the signal receivedby the radio unit; and a second selection control section configured toselect, as the hand-off destination candidate, one of the firstperipheral base stations which satisfies the preset condition, based onthe communication quality measured by the second measurement controlsection.